This application describes studies aimed at elucidating the role of the HIV-1 Nef protein. The conservation of this gene product in primate lentiviruses, as well as experiments done in the SIV system, indicate that Nef is essential for the persistence of these viruses in the infected host, and for the pathogenesis of AIDS. Yet, the mechanisms of action of Nef remain a mystery, hampering the rational development of therapies which could target this viral protein. In the past, a number of studies have evaluated the effects of Nef on the in vitro growth of HIV. These experiments have generated a plethora of contradictory results, triggering an often bitter scientific controversy. Novel approaches are obviously needed to understand the function of Nef. This application proposes to concentrate on the effects exerted by Nef on the T helper cell, the main target of HIV. Our and other people's work show that Nef dramatically downregulates the cell surface expression of CD4, which is the main HIV receptor, but also plays a major role in immune recognition. The study of this phenomenon will be used as a guide to explore the roles of Nef, its potential ability to interfere with T cell function, and its influences on the life-cycle of HIV. The planned experiments will address the following questions: 1. What are the mechanisms of Nef-induced CD4 downregulation? 2. Does Nef influence the response of T cells to antigens or other stimuli? How? 3. What are the functional domains of Nef? 4. How does Nef-induced CD4 downregulation influence reinfection and envelope-CD4 interactions?